This invention relates to a process for forming a metal or metal compound coating on a face of a heated glass substrate during its displacement in a given direction through a coating station by contacting the substrate with at least one stream of droplets comprising a substance or substances from which said coating metal or metal compound is formed on said face. The invention also relates to apparatus for use in carrying out such a process.
Processes of the above kind are employed for forming coatings which modify the apparent colour of the glass and/or which have some other required properties in respect of incident radiation, for example an infra-red-reflecting property.
Problems are encountered in forming coatings which have uniform properties. This is partly due to the difficulty of ensuring uniformity of the structure and thickness of the coating from one zone to another.
In United Kingdom Pat. No. 1 516 032 a process of the kind referred to is described wherein the formation of an homogeneous coating is promoted by discharging the coating material against the substrate as a stream which is inclined downwardly towards the substrate, in the direction of its conveyance, so that the acute or mean angle of incidence of such stream on the substrate, measured in a plane normal to the substrate and parallel with its direction of conveyance, is not more than 60.degree..
United Kingdom Pat. No. 1 523 991 discloses a glass coating process of the same kind in which, for the same purpose of promoting the homogeneity and uniformity of the coating, suction forces are created in exhaust ducting situated so as to cause gases environmental to the droplet stream to flow away from said stream and into such ducting, substantially without affecting the paths of the droplets towards the substrate.
Even when observing the conditions proposed in the said prior patents, defects have sometime occurred below or at the surface of the coating which defects although often not very prominent, nevertheless disqualify the product from the top quality ratings which are now in demand. If the defects are at the surface of the coating, the quality of the product can in some but not all cases be improved by an after-coating surfacting treatment, but of course such additional treatments add to the product cost.
The residual defects are often not very marked but they may nevertheless disqualify the product from the top quality ratings which are now in demand. Taking into account the numerous factors which can influence coating quality and which can differ from one process to another, according to circumstances, it is not expected that any given quality control measure will be wholly satisfactory in all cases. But the present invention provides a control measure which has been found to be beneficial at least for reducing the incidence of the said coating defects.